Printing presses, such as those used for offset or dry offset printing, employ printing blankets usually formed of a heavy textile fabric backing coated with rubber, or preferably several plies of alternating layers of fabric and rubber. The blanket is wrapped about the periphery of an associated printing cylinder to pick up an ink design or image deposited thereon by an inked printing roller or plate for transfer onto a sheet of paper or other article to be printed with the design. The cylinder about which the blanket is wrapped is usually provided with a longitudinal recess along its periphery into which the ends of the blanket may be inserted and releasably anchored to so-called reel rods or reels extending longitudinally along and within the recess.
Because the blanket wears out and must be replaced frequently, the means of anchoring the blanket ends to the reel rods should permit quick attachment and detachment of the blanket ends. Consequently, such anchoring means have been the subject of extensive research efforts. Some of the anchoring means developed as a result of this research are disclosed in U.S. Pat. Nos. 1,140,511; 1,670,418; 2,249,938; 2,712,789; 2,850,970; 2,986,085; 3,166,012; 3,260,200; 3,296,673; 3,332,346; 3,489,085; and 3,563,176. A recently devised means of anchoring the blanket ends that has achieved a significant amount of commercial acceptance is disclosed in U.S. Pat. No. 4,090,444 to Stearns.
The Stearns anchoring means comprises a holding bar adapted for clamping a blanket end and adapted for insertion into a channel provided in a reel. FIG. 1 shows the Stearns holding bar clamping a blanket end and inserted into a reel channel, with the blanket being wrapped slightly more than three revolutions around the reel to tighten the blanket around the printing cylinder. This arrangement is discussed in column 5, lines 60-68 of the Stearns patent. Since the holding bar protrudes from the channel above the surrounding reel periphery, the blanket is subjected to a localized, relatively great tension in the areas immediately adjacent to and generally radially above the holding bar. The tension causes uneven compression of the blanket and thereby permanently mars and deforms the blanket in those areas. Although such deformities do not significantly affect the operating life of single use blankets, the deformities gravely affect the operating life of multiple use blankets, effectively prohibiting the use of a Stearns type holding bar with multiple use blankets.
Multiple use printing blankets are becoming increasingly and extensively used, especially in printing metal cans for containing beverages. Basically a multiple use blanket is a blanket that is several times longer than the circumference of its associated printing cylinder. For example, the blanket might be approximately sixty inches long and its associated printing cylinder might have a circumference of about eighteen inches. Initially the leading blanket edge is attached to a first reel within the printing cylinder, and the trailing edge is attached to a second reel within the printing cylinder. The second reel is then rotated several times to tighten a portion of the blanket about the printing cylinder, thereby winding a portion of the blanket several times around the second reel. When the blanket portion initially wrapped about the printing cylinder is worn, a fresh, unused blanket portion may be wrapped about the printing cylinder by rotating the first reel to wind the worn blanket portion therearound and by rotating the second reel to unwind part of the blanket portion therearound, whereby the outermost blanket layers wound about the second reel become the fresh, unused blanket portion wrapped about the printing cylinder. This process may be repeated until the entire blanket is worn.
Multiple use blankets are also used in so-called segmented printing cylinders wherein the cylinder is provided with several longitudinal recesses equiangularly spaced about the cylinder periphery, each recess having a pair of reel rods extending therealong and therewithin. The multiple use blanket is connected to the adjacentmost reel rods of adjacent recesses and is wrapped about the printing cylinder periphery between the adjacent recesses. The blanket is wound and unwound in much the same manner as described above, however, in this latter case, the multiple use blanket is several times longer than the printing cylinder periphery between adjacent recesses, rather than the entire printing cylinder periphery.
Since the time involved in winding a fresh blanket portion about the printing cylinder is much less than the time involved in replacing a blanket, one can readily appreciate that multiple use blankets are strongly preferred over single use blankets. One can also appreciate that use of a Stearns type holding bar with multiple use blankets creates deformities in blanket portions that are eventually wrapped about the printing cylinder. These blanket portions are effectively useless for printing since, as is well known in the printing art, even minute blanket thickness variations amounting to even fractional thousandths of an inch produce very large changes in the color or intensity of the printed image.